Animportant,althoughrelativelyuncommon,conditionthatcaninterruptthefunctionofcochlearnervefibersisanacoustic neuroma, atumorofSchwanncellsofthevestibulocochlearnerve(CNVIII).Asthetumorgrows,irritationofcochlearnervefibersmaycausearingingsoundintheaffectedear(tinnitus). Eventually,conductionincochlearnervefibersisblocked,andtheearbecomesdeaf.Thetumormaybeoperablewhilestillsmall;therefore,earlydiagnosisisimportant.Ifthetumorisallowedtoenlargesubstantially,itcouldinterrupttheentirevestibulocochlearnerveandcausevestibularaswellasauditorydifficulties.Itcouldalsoimpingeonordistortneighboringcranialnerves(e.g.,V,VII,IX,andX),anditcouldproducecerebellarsignsbycompressingthecerebellarpeduncles.
•Fig. 8.22 A, Atlowfrequencies,individualauditoryafferentfiberscanrespondateachcycletothesignalfrequency.B, Athigherfrequencies,eachafferentfibergeneratesanactionpotentialonlyatcertaincycles,limitedbyitsmaximumfiringfrequency.However,theoverallpopulationofafferentfiberscanstillsignalstimulusfrequencybytheiraggregatefiringfrequency.
Cochlear afferent fibers synapse on neurons of the dorsal and ventral cochlear nuclei. The neurons in these nuclei have axons that contribute to the central auditory pathways. Some of the axons from the cochlear nuclei cross to the contralateral side and ascend in the lateral lemniscus, the main ascending auditory tract. Others connect with various ipsilateral or contralateral nuclei, such as the superior olivary nuclei, which project through the ipsilateral and contralateral lateral lemnisci. Each lateral lemniscus ends in an inferior colliculus. Neurons of the inferior colliculus project to the medial geniculate nucleus of the thalamus, which gives rise to the auditory radiation. The auditory radiation ends in the primary auditory cortex (Brodmann areas 41 and 42), located on the superior surface of the temporal lobe.
The input from each ear is bilaterally represented in the ascending auditory system pathway at the level of the lateral lemniscus and above. Thus the representation of auditory space is complex, even at the brainstem level. Consequently, unilateral deafness may occur with isolated lesions of the cochlear nuclei or more peripheral structures. Central lesions do not cause unilateral deafness, although they may interfere with overall sensitivity to speech or with sound localization.
Functional Organization of the Central Auditory System
The responses of neurons in several structures that belong to the auditory system can be described by tuning curves (see
Fig. 8.21B ). By plotting the distribution of the characteristic frequencies of neurons within a nucleus or in the auditory cortex, a tonotopic map may be revealed in which neurons are ordered according to their “best” frequencies. Tonotopic maps have been found in the cochlear nuclei, superior olivary complex, inferior colliculus, medial geniculate nucleus, and auditory cortex. A given auditory structure may, in fact, contain several tonotopic maps.
Most auditory neurons at levels above the cochlear nuclei respond to stimulation of either ear (i.e., they have binaural receptive fields). Binaural receptive fields contribute to sound localization. A human can distinguish sounds originating from sources separated by as little as 1 degree. The auditory system uses several clues to judge the origin of sounds, including differences in the time (or phase) of arrival of the sound at the two ears and differences in sound intensity on the two sides of the head.